Facet tracking ROS systems are generally well known in the art. The input beam of a laser source is deflected by an acousto-optic (A-O) cell in such a manner that the light beam moves to "track" the effective mirrored facet on a rotating polygon. In addition to deflection, the A-O cell may also modulate the input beam in response to a video data signal input.
As is also well known, the Bragg condition must be satisfied in order for the light to be diffracted by the A-O cell. Briefly, the Bragg condition states that the sine of the angle defined by the input light beam and the acoustic plane wave in the A-O cell must be equal to a multiple of the wavelength of light divided by two times the wavelength of the acoustic wave. If the angle of the incident light does not satisfy the Bragg condition, then the light will pass through with only partial diffraction.
In practice, the light that is at an incident angle close to the Bragg angle will be mostly diffracted. In most ROS systems, however, the angle of incidence for the input beam is constant. Only the frequency of the acoustic wave varies; thus varying the Bragg angle. As a result, there is one acoustic frequency that gives the optimal diffraction (i.e. where the Bragg condition is exactly satisfied) and a narrow band of frequencies, close to the optimal frequency, where light is partially diffracted, albeit at different angles of deflection.
One desirable result of this arrangement is that the differences in angle of deflection allow the beam to track the facet. A second result is that the partial diffraction of light reduces the power of the beam leaving the A-O cell. This second result is undesirable from the standpoint of print quality. For example, uneven spot power incident upon a photoreceptor ultimately places a limit on the latent image contrast of the xerographic system. This is especially true if photoreceptors are potentially sensitive to the variations in spot power that occurs due to diffraction inefficiencies.
One attempt to correct this spot power differential due to diffraction inefficiency has been to "steer" the sound beam so that it maintains the Bragg angle with the incident light as the acoustic frequency is varied. The steering is accomplished by generating the acoustic wave with a phased array of multiple transducers coupled to the single A-O cell. This approach, however, is costly and complex as it requires additional electronics to drive the multiple transducers to accurately steer the sound wave.
Thus, there is a need to compensate for the power variation from the output beam from an A-O cell due to the variations in diffraction efficiency without the use of multiple transducers in an A-O cell.
It is therefore the object of the present invention to provide a means for controlling the spot power of the output beam without adding to the complexity of the optical system.
It is yet another object of the present invention to provide a means for having any desired output spot power at any time during the scan.